Electron scattering and charged pion photoproduction at 200MeV on 12C and I3C have been studied by the nuclear shell model to investigate M1 and E2 nuclear form factors of the isovector type. Starting with the Op-shell model of Hauge-Maripuu, the effects of core polarization by the M3 Y force and the one-pion-exchange current are taken into account. The effect of core polarization is found to be essential in both electron scattering and pion photoproduction, while the meson-exchange current, which also improves the calculated results, gives contributions of about 10 ~-15 ~.
Electron scattering and charged pion photoproduction at 200MeV on 12C and I3C have been studied by the nuclear shell model to investigate M1 and E2 nuclear form factors of the isovector type. Starting with the Op-shell model of Hauge-Maripuu, the effects of core polarization by the M3 Y force and the one-pion-exchange current are taken into account. The effect of core polarization is found to be essential in both electron scattering and pion photoproduction, while the meson-exchange current, which also improves the calculated results, gives contributions of about 10 ~-15 ~.
“…Further, if the single particle elements, also given in Table 4, are used to estimate the isovector B(E2) value, strong cancellations occur and predictions then are very sensitive to details of the spectroscopy. As a consequence, our PHM calculation gave a value of O' 02 e 2 fm 4 for this isovector B(E2) value, whilst the naive model yielded a value of 5· 85 e 2 fm4; a value also predicted by an RPA calculation (Friebel et al 1978). The suppression of transition strength is further displayed in Fig.…”
The energy level spectrum of 12C is calculated in a truncated but large shell model space of projected one particle-one hole Hartree-Fock determinants using a realistic g matrix. Predictions of electromagnetic decays and electron scattering form factors are compared with experimental values.
“…These states are * To exhibit the characteristic of 12C spectrum and for sake of clarity we show in Fig. 4 for a given jr and T only states with small excitation energies [8]; obviously, they are very sensitive to details of the corresponding wave functions [21]. From an inspection of the model wave functions of the 1 + and 2 + states it follows, that the main component is the (lpt/2 lp3/2) (ph)-configuration (about 99~; see Table2) with small admixtures of (2h~o) components.…”
Section: Results Of the (Lp 1 H)-caleulationsmentioning
confidence: 99%
“…However, a unifying and comprehensive theoretical description of the nuclear structure of 12C over a large range of excitation energies within a large configuration space seems to be of actual interest. Only within such an extended model recent experimental data might be interpreted consistently -the radiative capture of protons with an energy around 100MeV to high lying states in 12C [20] shows a strong evidence for nuclear transitions between configurations with excitation energies of (i h~o) and (2h~o), a finding which is of significant importance for the extraction of high lying single particle states; -high-resolution electroexcitation experiments [21,22] to the 2 + states at 4.439MeV and 16.107MeV in ~2C have presented detailed information about the spatial distribution of nuclear convection and magnetization currents; -recent high-resolution inelastic pion scattering [23] at intermediate energies to many excited states in ~2C below 30 MeV has stimulated the interest on a detailed and consistent description of the ~2C spectrum; -a comparison of recent (e,e') data on low lying non-normal parity states in HB [24] with theoretical estimations [25] clearly maps out a large influence of the ls-hole component in the states involved.…”
In a consistent shell model approach excitation energies and electromagnetic transitions are calculated for the low energy part of the 12C spectrum up to the giant resonance region within a (lp lh+2p2h) configuration space. Comparison with available experimental data shows that almost the complete spectrum up to 40 MeV can be described consistently. Thereby it is found that whereas the low lying collective states remain nearly unchanged by the inclusion of (2p2h) configurations, the spreading widths of all (2he o) giant resonances are influenced drastically. For serious discrepancies in a few levels (4p4h) configurations or mesonic and baryonic degrees of freedom might be responsible.
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